Battery based voltage references are commonly used in instrumentation that requires a floating reference or offset voltage. Some instruments rely on a battery reference to establish a basis for absolute measurement accuracy. The voltage reference must have high ohmic and impedance isolation from the instrument ground to avoid errors or circuit damage that can occur from ground loops and loading effects.
Alternatives to batteries have been investigated. Solid state voltage sources are available but it is questionable if they can meet the isolation and stability specifications provided by batteries, specifically mercury cells. Solid state isolation methods are typically based on transformer or capacitor coupled technology. Although these methods provide suitable ohmic isolation, they introduce errors from leakage currents and from capacitive impedance coupling. These limitations often cause the designer to rely on battery reference techniques.
Since batteries have a limited service life, it is recognized that they are not the optimal solution. With batteries, high stability is maintained until cutoff is reached; at which point, the voltage output gradually decays. If the battery is not quickly replaced, offset errors are introduced. This can pose a problem since batteries are often embedded in the instrumentation and are inconvenient to monitor and replace. The disposal of used batteries presents an additional problem due to increased costs associated with disposal and potential environmental impacts. Mercury batteries, which are often used because of their high stability, are considered hazardous waste.
Advances in technology have made it possible to design solid-state voltage reference systems suitable to replace batteries. By using optical isolation techniques, it is possible to reduce errors from leakage currents and impedance coupling. Pulse rate frequency modulation is used to eliminate nonlinearities in the optical transmission link which occur from normal age-related intensity changes in the light source. The use of phase-locked loop feedback in the demodulation stage provides a highly stable frequency to voltage transfer function.
Accordingly, it is an object of this invention to provide an alternative to batteries which can deliver the isolation and stability inherent in batteries.
A further object of this invention is to provide a system which employs optical isolation techniques to reduce errors due to current leakages and impedance coupling.
A further object of this invention is to provide for a modulating technique to eliminate nonlinearities in the optical transmission.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of instrumentalities and combinations particularly pointed out in the appended claims.